Figure REF  schematically illustrates the layout of the exchange-based surface-code architecture proposed in this work. The architecture, as its predecessor dipole-based architecture [1]}, is based on three-dimensional layout. In Figure REF  (a) The colored dots indicate 2D arrangement of donor atoms, interleaved with black squares representing SET islands for loading/unloading and readout of electron to/from qubits. The nuclear spins on donors define the qubit states as shown in Figure REF  (b). The 2D qubit plane is sandwiched between the top and bottom layout of wires forming source and drain. The exponential decay of the exchange interaction with the separation between the donor atoms is well known in the literature, as is the sensitivity of the interaction to valley interference effects [2]}, [3]}, [4]}, [5]}, [6]}, [7]}, [8]}, [9]}, [10]}, [11]}, [12]}, [13]}, [14]}, [15]}, [16]}, [17]}, [18]}. This results in a tension between donor separation and exchange strength to design a fast CNOT gate while maintaining sufficient distance between the atoms to allow for control wires, also known as pitch problem. In the previous dipole-based architecture [1]}, the separation between the adjacent donor atoms was taken to be 30 nm, defined by the gate-leakage pitch limit for STM control-lines. At such distances, the exchange interaction is effectively zero. In our scheme we introduce a coupler donor which switches the exchange on and off by loading and unloading an electron to that position (Figure REF  (c)).
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